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In the case of red blood cells: human erythrocytes (red blood cells) have no mitochondria. Since the mitochondria are the cellular site for oxidative metabolism of fatty acids, erythrocytes cannot oxidise fatty acids to release energy. The erythrocytes also cannot fully oxidise glucose (to carbon dioxide and water) because this is also a mitochondrial process, so they have to rely upon anaerobic glycolysis. The end product of anaerobic glycolysis is pyruvate, and erythrocytes reduce this to lactate (to recycle the NADH that is produced during glycolysis) and then export this lactate into the blood for further metabolism by the liver.

The brain and heart can take advantage of ketone bodies when the amount of glucose is low. These are byproducts of fat metabolism and can be converted to acetyl-coA via the citric acid cycle.

Overproduction of these products can cause pathological conditions:

When the rate of synthesis of ketone bodies exceeds the rate of utilization ,their concentration in blood increases , this is known as ketonemia. This is followed by ketonuria- excretion of ketone bodies in urine. The overall picture of ketonemia and ketouria is commonly referred as ketosis. Smell of acetone in breath is a common feature in ketosis.